1. Field of the Invention
The present invention relates to transducers, and particularly, to improved ultrasound transducer arrays.
2. Description of the Related Art
Ultrasound machines are used to non-invasively obtain image information about the structure of an object which is hidden from view and has become widely known as a medical diagnostic tool. As shown in FIG. 1, medical ultrasonic transducer arrays conventionally are fabricated from a block of ceramic piezoelectric material within which individual elements are defined and isolated from each other by sawing at least partially through the block of piezoelectric material making a number of cuts with a dicing saw. In particular, as shown in FIG. 1 a transducer array 100 includes a layer 104 of transducer elements 104a-104f. The transducer elements 104a-104f are laid down on a backing medium 102. The backing medium 102 serves to support the transducer structure. One or more acoustical matching layers 106 (including elements 106a-106f) and 107 (including elements 107a-107f) may be laid down on top of the transducer layer 104. The piezoelectric layer 104 may be formed of any piezoelectric ceramic material such as lead zirconate titanate (PZT). The matching layers 106, 107 and the transducer layer 104 may be glued to one another using an epoxy, such as Der332. The layers are then diced by forming kerfs 110a-110e with a standard dicing machine. Typically the kerfs 110a-110e are made both in the direction parallel to the paper and perpendicular to the paper.
Typically, the ratio of the width to the thickness of the piezoelectric elements 104a-104f is optimized to about 0.5. The ratio of the width to thickness of the matching layers is typically ignored. As is generally known, the basic requirement for the transducers is high bandwidth, low pulse length, low crosstalk to the neighboring elements. However, if the width and thickness ratio of the matching layer is close to 1, lateral and thickness vibration mode will have a much stronger coupling, which in turn, will provide higher crosstalk, and unpredictable spectrum and pulse, which can degrade image quality.
Conventionally, the elements are sub-diced in order to change the width and thickness ratio of the ceramic piezoelectric material. For example, for the Siemens 5L40 transducer, the element is sub-diced once so that each sub-element width is around 116 micrometers and the thickness of the PZT element is about 256 micrometers. The resulting ratio of about 0.46 for the piezo-active layer results in a very good value for KT (electromechanical coupling coefficient) and low coupling. However, for the matching layer the thickness is typically about 130 micrometers resulting in a ratio of about 0.91, leading to a relatively strong coupling between the thickness mode and the unwanted lateral mode.
These drawbacks of the prior art are overcome in large part by an ultrasound transducer array according to the present invention. According to one embodiment of the invention, the piezoelectric elements and the matching layer(s) are diced with different sub-dicing. In particular, according to one embodiment the PZT is sub-diced once but at the same time the first matching layer is sub-diced twice to obtain a more optimum ratio for the matching layer.